This application claims the priority of the European patent application No. 01104544.0, filed Mar. 5, 2001 the disclosure of which is incorporated herein by reference in its entirety.
The invention relates to a method for the structural or high grade, respectively, bonding of painted bodies such as car bodies or modules, in particular painted metal modules or plastic modules.
In vehicle construction the tendency goes towards light weight construction, the production of larger modules as well as the enhancement of the tortional stiffness of the car body or thereto mounted parts such as e.g. doors, front flaps and rear flaps etc. An essential requirement therefor is the application of adhesives instead of welding points and sealants. In future, welding points will almost only be needed for the positioning of the parts (dimensional accuracy) during production. The growing desire aims at adhesives that structurally or frictionally, respectively, join the modules with high strength transfer, and at bonded structures that behave like unitary parts.
The structural bonding in car body construction where high grade adhesives (so called carcase work adhesives) are applied to oiled metallic substrates, has already achieved a high level of quality. Today already so called crash resistant carcase work adhesives are known that upon a crash do not lead to a rupture within the adhesive, but that are able to force the composite to fold.
Contrary thereto the structural bonding during assembly, where varnished metal surfaces are at issue, is not yet solved to satisfaction, since the build up of the lacquer itself can not take over the structural forces. The build up of the lacquer usually consists in the following layers: after degreasing, the metal surfaces are phosphated, coated by means of cathodic dipping finishing (CDF) and annealed in an oven at high temperatures around 180xc2x0 C. during about 30 minutes. Said CDF layer is responsible for the protection against corrosion. In the following finishing process, a xe2x80x9cfillerxe2x80x9d and a basic varnish, and optionally furthermore a coating lacquer, are applied that are dried in a re-circulated air oven at temperatures around 130xc2x0 C. during about 20 minutes. Practice has now shown, that the adhesion of the CDF to the phosphate layer or also of the xe2x80x9cfillerxe2x80x9d to the coating lacquer can fail due to high stress and therewith are the strength limiting factors. Furthermore, in the lacquer layer a predetermined braking point is provided to insure that in case of e.g. falling stones, the damage is only within the cover lacquer itself (now visual colour changes) and not between xe2x80x9cfillerxe2x80x9d layer and basic varnish. Said predetermined braking point only admits a bonding with a modulus of shear of about 5 MPa.
According to the state of the art the following method is applied e.g. for the bonding of panes. Either the bonding is performed directly on the cover lacquer, or the CDF coated flange of the car body is masked with an adhesive tape or with an organic composition such as e.g. PVC plastisol. The plastisol is e.g. gelled by means of an IR-radiator and thereby achieves solid consistency. At the end of the finishing process and prior to the pane assembly the flange masque usually is manually removed and the direct glazing performed on the CDF. The method as described above does not allow a reliable bonding with high structural forces, since at shear forces  greater than 8-10 MPa the adhesion of the CDF layer fails.
The present invention therefore aims at an improved method that surpasses the limitations of the state of the art and enables structural bonding of  greater than 8 MPa in final assembly.
Hence, it is a general object of the invention to provide a method for the bonding, in particular the structural bonding, on surfaces after the application of a lacquer, wherein the surface is coated with at least one structural polymer at least in the region of bonding, and wherein said surface is provided with at least one lacquer layer, and wherein said at least one lacquer layer is removed from the at least one structural polymer prior to the application of an assembly adhesive.
The surface to be bonded usually is a metal surface or a plastic surface,